This invention relates to improvements for electrical amplifiers. More specifically, this invention relates to circuits and methods for creating amplifiers with improved frequency response characteristics.
Amplifiers are virtually ubiquitous in modern electronic systems. The necessity for amplifiers, and the amplification that they provide, arises because electric signals of interest are often too “weak” for reliable acquisition and subsequent processing. Such operations, however, may be made more reliable by increasing a subject signal's magnitude.
One desirable characteristic of amplifier circuitry is the ability to provide linear amplification (i.e., amplification that allows signal information to be preserved by amplifying the original signal by a known gain). Often, the amplified output signal is identical to the input signal, with the exception of having a larger magnitude and in some instances, phase shifted. Any alteration of the waveform, however, aside from a change in magnitude or phase, is typically considered a distortion of the original signal and frequently produces undesirable results such as quantization errors when creating digital signals. Accordingly, in many applications, a substantially constant signal gain with minimal variation is highly desirable.
For other types of amplifiers, however, such as limiting amplifiers, it is important to non-linearly amplify the input signal, and thereby produce an output that preserves the timing characteristics of the input signal, but limits signal amplitude. Such amplifiers are commonly employed in high speed transmission systems for digital data. Maximum bandwidth and minimum rise time are desirable to permit the highest possible data rate. Constant gain is desirable so that the input sensitivity is high and the timing variations are small.
Amplifier bandwidth is also of interest to circuit designers. The bandwidth of an amplifier usually signifies the range of frequencies over which the gain for an input signal is substantially constant (or as constant as possible). Generally speaking, the magnitude response of an amplifier resembles a parabola with negative concavity, including a flattened top (nearly a horizontal line at the vertex) between two frequencies, ω1 and ω2. Signals whose frequencies are below ω1 or above ω2 will experience lower gain, with the gain decreasing as the frequency moves farther away from the range defined by ω1 and ω2. The gain of the amplifier over the frequency band defined by ω1 and ω2, however, should remain substantially constant. This range of frequencies is referred to as the amplifier bandwidth.
Typically, electrical systems are designed so that the amplifier's operational bandwidth coincides with (or at least partially with) the spectrum of the signals it is designed to amplify. If this condition is not met, amplifiers often distort the frequency spectrum of the input signal, by amplifying different portions of the input signal by different amounts. Accordingly, a large bandwidth, which allows accurate amplification of signals over a wide range of frequencies, is particularly desirable.
One reason conventional amplifiers have frequency limitations is because various components associated with the amplifier respond differently at different frequencies. For example, capacitors and inductors employed in input or feedback networks almost always have frequency dependent reactances. Therefore, special care must be taken to avoid using the amplifier significantly outside of its predetermined bandwidth.
Currently, there are a variety of methods and circuit configurations for amplifying electrical signals. One commonly used topology is the complementary metal-oxide semiconductor (CMOS) amplifier. CMOS amplifiers use a combination of N-channel metal-oxide semiconductor field-effect transistors (MOSFETS) and P-channel MOSFETS in order to produce an amplified signal.
In the past, CMOS and other types of amplifiers have been introduced that make use of various electrical loads in addition to impedance matching. Although such techniques have been capable of providing either relatively large bandwidths or reduced gain variation, both of these benefits could not be achieved simultaneously. Additionally, the rise time associated with these techniques has been far from optimal.
It light of the foregoing, it would therefore be desirable to provide improved amplifiers with extended bandwidth and substantially constant gain. It would also be desirable to provide amplifiers with improved rise time.